1. Field of the Invention
This invention relates to an apparatus and method for measuring the resistance of superconductors. Structures of this type, generally, allow the resistance of the superconductor to be accurately measured in a non-destructive manner by using a bifilar coil which includes an integrated loop/switch formed from the bifilar coil.
2. Description of the Related Art
The use of superconductors for production devices has a problem in that the resistance (or absence thereof) of a long length of material cannot be easily measured nondestructively, i.e. without actually building the device and measuring its field accurately over time. The reason for this is that in producing the desired field, the current in the conductor also produces large forces on the conductor which require the conductor to be potted in a hard material, such as epoxy to avoid motion and consequent quenching of the superconductor.
One way to address this issue is to wind a bifilar coil, wherein two conductors are wound adjacent to one another so that current will flow in opposite directions when the winding is excited. The field thus produced is very low and the coil can typically be wound "dry" without concern about the forces, the resultant motion and quenching. This facilitates unwinding after a successful test and subsequent rewinding into the desired field-producing coil configuration. The bifilar coil can then be excited from its end leads and the voltage across the leads measured at a given current to infer the resistance of the conductor.
The measurement which is desired from such a screening test is one of the overall resistance of the length of superconductor together with any joints which may be present in it. Unfortunately, the resistance which is required for a persistent current device such as an MR magnet is very low, and therefore difficult to detect by voltage measurements. An estimate of the resistance level which must be attained for a given device can be made from an equation governing the behavior of a circuit with a series resistance (R) and inductance (L) (FIG. 1). For a given initial current I.sub.o at time 0, the current at time t is given by EQU I=I.sub.o e.sup.-tR/L (Eq. 1).
For example, the specification for field drift rate for an MR magnet is typically 0.1 ppm/hr. Solving Equation 1 for R/L with this desired current drift rate (the current and field are linearly related) yields R/L=2.8e.sup.-11 .OMEGA./H. For a magnet with an inductance of 20 Henries, the overall coil resistance must be no greater than 5.6e.sup.-10 .OMEGA.. At a current of 150 A, the voltage generated by such a resistance is 83e.sup.-9 V. Because of thermally induced voltages in the sensing leads and at junctions, as well as, other effects, the accurate measurement of such a low voltage is very difficult-a typical limitation on accurate voltage measurement is perhaps 100e.sup.-9 V. Therefore, inference of resistance of the sample of superconductor from the measured voltage can lead to results which are not of the required accuracy.
Resistance tests of superconductors have been made in the past by measuring the voltage at a given current for long (up to 70,000 feet) and short (4-12 inches) lengths of conductor. Resistance has been measured from drift tests on individual loops of conductor, but this qualifies only that short length of material and is not useful for qualifying lengths which are required to wind a complete magnet. Therefore, a more advantageous system, then would be presented if the resistance of long lengths of superconductor could be measured in a non-destructive manner.
It is apparent from the above that there exists a need in the art for a system which measures the resistance of a superconductor, and which at least equals the voltage measurement accuracy of the known measurement systems, but which at the same time is capable of measuring long lengths of superconductor. It is a purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.